1. Field of the Invention
The present invention relates to a fixing device and a fixing temperature control method for image forming apparatuses such as copying machines and various types of printers. More particularly, the invention relates to a fixing device having a paper entry guide disposed below and in close proximity to an endless fixing belt which is passed around a heating roller and a fixing roller and conveys recording paper to a nipping portion composed of the fixing roller and a pressing roller in order to achieve image fixing by sandwiching the recording paper between the fixing roller and the pressing roller via the fixing belt, and also relates to a fixing temperature control method for use with such a fixing device.
2. Description of the Related Art
Traditionally, for an electrophotographic-type fixing device, a heat roller method has often been used wherein a pair of upper and lower rollers coated with a nonadhesive material such as a fluorine resin or silicone rubber and containing therein a heat source such as a halogen lamp are pressed together with a suitable pressure and image fixing is performed by passing toner-transferred recording paper through the nipping portion composed of the two rollers.
Since the heat roller method requires that the toner and the recording paper be rapidly heated to a fixable temperature using only the nipping portion composed of the pair of upper and lower rollers pressed together, if a fixing construction capable of a satisfactory transport speed is to be achieved it has been necessary to make the outer diameter of the heat roller large enough to provide the necessary heat capacity, and to use a considerably high fixing temperature; furthermore, to prevent the resulting high temperature offset, application of a large quantity of separation oil has been indispensable.
With this method, however, it has been difficult to shorten warm-up time or reduce power consumption, and there has also been the problem of image smearing, etc. with the separation oil.
As a technique having the potential of being able to solve the above problem, a belt-type fixing device has recently been proposed in Japanese Unexamined Patent Publication JP-A 6-318001.
This belt-type fixing device will be described below with reference to FIGS. 22 and 23.
The fixing device comprises an endless fixing belt 44 stretched around a heating roller 42 with a heat source 41 contained therein and a fixing roller 43, a pressing roller 45 for pressing the fixing roller 43 from below via the fixing belt 44, and a paper entry guide 46 formed in a substantially straight shape and disposed below and in close proximity to the fixing belt 44 and upstream of a nipping portion composed of the fixing belt 44 (fixing roller 43) and the pressing roller 45. The surface temperature of the fixing belt 44 is detected by a temperature sensing element 47 disposed in contacting relationship with the surface of the fixing belt 44 on the heating roller 42 side of the fixing belt 44, and the heat source 41 is controlled on and off using the sensor signal and a controller mounted in the main body (not shown). The fixing belt 44 is a low heat capacity belt constructed from a thin-film endless metal belt coated with a nonadhesive silicone rubber or the like.
When recording paper with an unfixed toner image formed thereon is transported :into the thus constructed fixing device, the toner and the recording paper are preheated through the fixing belt 44, and the toner begins to melt. Accordingly, temperature can be set low for the nipping portion composed of the fixing belt and the pressing roller; also, since the heat capacity of the fixing belt 44 is small, the fixing belt 44 is rapidly cooled when the recording paper passes the nipping portion, which makes it possible to reduce the temperature of the toner on the recording paper separated from the fixing belt 44 at the exit of the nipping portion, the net effect being that an offset-free good quality image can be obtained without using oil or by using a minute quantity of oil.
FIG. 24 is a diagram showing the temperature of the fixing belt on the heating roller side and the temperature of the paper entry guide as a function of time during warm-up from main body power on in the prior art construction.
In the above fixing device, however, sufficient study has not been made considering the setting environment of the main body and double-sided print mode.
More specifically, since the paper entry guide is disposed below and in close proximity to the fixing belt, the leading edge of the recording paper may become wavy or curled when operating in a high temperature, high humidity environment or when printing on the reverse side of the paper in double-sided print mode, as a result of which the leading edge of the recording paper after image transfer may not be inserted into the narrow space between the paper entry guide and the fixing belt, resulting in a recording paper jam, or a recording paper jam may occur during printing operation, causing the main body to stop its operation with the jammed paper squeezed between the paper entry guide and the fixing belt. If this happens, the user has to remove the jammed paper inside the main body and has difficulty in removing the jammed paper squeezed in the narrow space between the paper entry guide and the fixing belt. This has worsened recoverability from paper jamming.
One way to solve the above problem would be to preset the spacing between the paper entry guide and the fixing belt wider, but this would defeat the purpose of preheating the paper entry guide from the fixing belt (to improve the fixing characteristic by raising the temperature of the unfixed toner on the recording paper utilizing the heat contained in the paper entry guide). Another problem is that, in the case of a large heat capacity recording paper such as a thick paper, it is difficult to obtain a good fixing temperature by preheating from the paper entry guide alone.